well they appear @ random, however to increese your chanses of getting one... a shiny wurmple 4 eg. go to a place that there is alot of them and roughly on the 100th one it might be a shiny(i have a shiny groudon)

(\ /)(O.o)(> This is Bunny. Copy Bunny into your signature to help him on his way to world domination.MY FC IS: 0130 1538 1479